1. Field of the Invention
The present invention relates to digital-to-analog (DA) conversion circuitry incorporating therein a simplified testing circuit for testing the function of a DA converter or converters included in the circuitry.
2. Description of the Background Art
Conventionally, there is a type of DA conversion circuitry which is adapted to receive test data in a digital form on its test data port from a tester jig in a test mode designated by the jig and produce an analog voltage corresponding to the test data from its analog output port, which is in turn connected to a voltmeter indicating the analog voltage. More specifically, when a mode signal provided from the tester jig indicates the test mode, a selector included in the conversion circuitry selects the test data port and transfers the test data to a DA converter to be tested which is also included in the circuitry. The DA converter in turn outputs an analog voltage corresponding to the test data. A buffer amplifier connected to the DA converter amplifies the analog voltage and feeds the analog output port with the thus amplified analog voltage.
In this condition, the test data is fed from the jig to the DA converter via the selector. The voltmeter connected to the output port of the circuitry indicates the analog voltage derived from the test data. If one views the voltmeter and determines the analog voltage lying in its allowable range, then he or she increments the test data and measures again the resultant analog voltage on the voltmeter. In this manner, while the test data are sequentially incremented, the resultant analog voltages are compared with the test data for determining whether or not the DA converter satisfies its specifications.
As stated above, in the conventional DA conversion circuitry, analog voltages sequentially output from the circuitry are measured by the voltmeter one by one while being compared with the sequentially incremented test data. However, such a testing procedure is sophisticated and time-consuming.
It is an object of the present invention to provide DA conversion circuitry incorporating a simplified testing circuit that allows a DA converter or converters included in the circuitry to be easily tested in a shorter time.
In accordance with the present invention, DA conversion circuitry includes a selector for selecting test data input from the outside of the circuitry when a mode signal selectively designating a usual mode or a test mode designates the test mode. A DA converter converts the test data selected by the selector to an analog voltage. A voltage holding circuit holds the analog voltage output from the DA converter in response to a sampling signal input also from the outside and outputs it as a hold voltage. A comparator compares the analog voltage output from the DA converter with the hold voltage and outputs a signal representative of a result of comparison. The comparator may be replaced with a differential amplifier circuit that outputs a voltage substantially equal to a difference between the analog voltage output from the DA converter and the hold voltage.
Also, in accordance with the present invention, DA conversion circuitry includes a first selector configured for selecting first test data input from the outside of the circuitry when a mode signal selectively designating a usual mode or a test mode designates the test mode. A second selector selects second test data also input from the outside when the mode signal designates the test mode. A first DA converter converts the first test data selected by the first selector to a first analog voltage while a second DA converter converts the second test data selected by the second selector to a second analog voltage. A differential amplifier circuit outputs a voltage substantially equal to a difference between the first and second analog voltages output from the first and second DA converters, respectively.